The field of application of the invention is the measurement of heart rate frequency, i.e. pulse, and especially the calculation of heart rate on the basis of an EKG signal measured from hands.
A heart rate monitor is an apparatus used in sports and medicine, by means of which it is possible to monitor the functions of the heart, such as heart rate, standard deviation of heartbeat intervals, or other corresponding heart rate information derived from the functioning of the heart. There are two types of heart rate monitors which differ from each other in principle. Heart rate monitors of the first type measure heart rate frequency optically from the flow of blood in blood vessels or heart rate as pressure measurement from blood vessels. Typically, optical measurement and pressure measurement are done from hands, from wrist, for instance. Heart rate monitors based on the measurement of an EKG signal are in the second category of heart rate monitors. The EKG signal is strongest on the chest of a person where the most reliable measuring results are obtained, but especially in sports applications, areas providing weaker electric signals, such as wrists and palms, are used in signal measurement for reasons of comfort and practicability.
Many kinds of solutions for heart rate monitors are known. A structure based on a heart rate transmitter belt located on the chest and a wrist receiver can be mentioned as an example. Further, some heart rate monitor solutions are implemented solely by a wrist apparatus, in which case the heart rate measurement and processing occur in one unit. Instead of these portable heart rate monitor solutions, heart rate measurement functions can also be integrated in sports apparatuses, such as exercise bikes, steppers and treadmills. In these apparatuses, heart rate measuring sensors are located in the handles, i.e. heart rate is measured on the basis of an EKG signal measured from palms.
One of the most difficult heart rate measuring arrangements is measuring the EKG by means of measuring electrodes placed in the handles of an exercise apparatus. The measuring environment in question is especially difficult because unlike in the case of a wrist apparatus or heart rate transmitter belt, the measuring electrodes are not attached to the body. The movement of hands in relation to the handles and the electrodes located on them causes interference to the signal being measured, as does the EMG signal caused by the movement of muscles.
U.S. Pat. No. 5,738,104 discloses a prior art solution for EKG signal measurement and heart rate formation on the basis of the measured signal. The solution disclosed in the publication measures the EKG signal by means of at least three measuring electrodes and tries to locate an R peak of the QRS complex from the measured signal. U.S. Pat. No. 5,876,350 describes another prior art solution for calculating heart rate frequency on the basis of an EKG signal.
Prior art solutions may produce satisfactory results in good measuring conditions, but they do not, however, provide conditions for a successful heart beat detection in an environment comprising interference, such as in a measurement done from the handles of an exercise apparatus. During a fitness exercise, the EMG signal caused by the movement of muscles and the movement of hands in relation to the handles cause interference to the measuring signal, and the elimination of this interference appears insufficient on the basis of the solutions disclosed in the reference publications. Due to their inaccuracy, the solutions easily lead to incorrect detection, and consequently, the calculation of heart rate loses its reliability.
It is thus clear that there is a need to develop a method and an apparatus so as to achieve a sufficiently reliable heart beat detection and heart rate calculation in a measuring situation comprising a great deal of interference.
It is an object of the invention to provide an improved solution for measuring heart rate from hands. This is achieved by the following method for detecting a heart beat and calculating heart rate on the basis of the detected heart beats, the method comprising forming an EKG signal on the basis of signals measured from the skin of a person, detecting from the EKG signal a heart beat candidate to which a rationality analysis is performed, in which the time difference between the heart beat candidate and the heart beat detection preceding it is calculated and on the basis of the time difference, a momentary heart rate value is calculated, the calculated momentary heart rate value is compared with a median value of the heart rate, which is a median of two or more earlier heart rate values, the heart beat candidate is accepted as a heart beat detection, if the momentary heart rate value calculated on the basis of the heart beat candidate is, when compared with the median value, within threshold values of the median value, and updating as the heart rate value the momentary heart rate value calculated on the basis of the accepted heart beat detection.
The invention also relates to an arrangement for detecting a heart beat and calculating heart rate on the basis of the detected heart beats, comprising at least one measuring electrode for measuring a signal from the skin of one hand of a person, and at least one measuring electrode for measuring a signal from the skin of the other hand, an amplifier connected to said measuring electrodes for amplifying the potential difference of the signals measured by the measuring electrodes to form an EKG signal, an analog-to-digital converter for receiving the signal from the amplifier and for converting the signal into digital format to be a sample signal containing samples, and a computer which is arranged to read the digital signal and to detect from the sample signal a heart beat candidate, to perform a rationality analysis to the heart beat detection, in which the computer is arranged to calculate a time difference between the heart beat candidate and the heart beat detection preceding it, and to calculate a momentary heart rate value on the basis of the time difference, to compare the calculated momentary heart rate value with a median value of the heart rate, which is a median of two or more earlier calculated heart rate values, to accept the heart beat candidate as the heart beat detection, if the momentary heart rate value calculated on the basis of the heart beat candidate is, when compared with the median value, within threshold values of the median value, and to update as the heart rate value the momentary heart rate value calculated on the basis of the accepted heart beat detection, the arrangement further comprising display means connected to the computer and displaying the heart rate value calculated on the basis of the accepted heart beat detections.
Preferred embodiments of the invention are set forth in the dependent claims.
The invention thus relates to a method and an apparatus for detecting a heart beat from an EKG signal and calculating heart rate on the basis of the detected heart beats. In the solution of the invention, the measurement of the heart rate signal is preferably done from hands, as in the measurement from the handles of an exercise apparatus, in which the measurement is done from the skin of a palm or finger tip, for instance. Detecting the signal is considerably easier when measured from the chest than elsewhere on the body, which means that the problems related to the measurement are then completely different from measurement in a measuring area where the ratio of the interfering signal to the measuring signal is high. The calculated heart rate is displayed to the user by the display means of the exercise apparatus, such as a liquid crystal display.
In the solution of the invention, a heart beat candidate is searched from an EKG signal, and a rationality analysis is performed to it. In the analysis, it is checked that the heart beat detection is in temporal proportion to previous accepted detections in such a manner that the heart beat candidate can be accepted as a heart beat detection to be used in calculating heart rate. In examining accepted heart beat detections, a preferably short time-window, for instance three seconds, is used. A median value is calculated for the heart rate on the basis of the heart beat detections in the time window. Using a median provides the advantage that unnecessarily high or low values in the time intervals of the examination point, which may be related to incorrect detections, will not be taken into account in the calculation, contrary to a calculation which uses an average, for instance. The heart beat candidate is accepted as a heart beat detection, if the heart rate calculated on the basis of the candidate differs upwards or downwards from the median less than the threshold values, i.e. for instance 10 beats a minute. Otherwise, the tentative heart beat candidate is rejected as an incorrect detection. The background for median calculation is that the alteration rate of heart rate, whether increasing or decreasing, is limited by the human body and can be relatively accurately defined. Heart rate calculation is preferably done at least at two-second intervals.
In a preferred embodiment of the method of the invention, an electric signal is measured from a person""s skin at two measuring points, and a potential difference between the measured signals is formed by means of a differential amplifier. Signal processing operations are performed to the formed signal, such as analog filtering in which frequencies outside the frequency range of 5 to 40 Hz are filtered from the signal. The obtained measuring signal is converted into digital format to be a sample signal which is examined during the heart beat search process. In heart beat detection, a sample window is slid over sample signals, i.e. the sample signal being examined is read from the memory buffer or in real-time to the sample window, and from the other end of the sample window, the sample signal moves away from processing. The sample signal in the sample window is read one sample at a time until a local maximum value is detected in the sample signal. A local maximum is required to exceed a threshold value which is set in advance or modifiable according to existing conditions. A threshold value can be formed in an existing measuring situation to be, for instance, twice the average interference power. After finding the local maximum, the routine checks if a local minimum which is lower than a threshold value set for the local minimum, follows the local maximum. Further, a local maximum which exceeds a threshold value set for a second local maximum, must follow the local minimum.
A preferred embodiment of the invention examines the time difference of the above-mentioned local minimum to the first local maximum and the second local maximum or to at least one of them. If the time difference is within the threshold values set for the time difference, the detected heart beat can be tentatively accepted.
The method and apparatus of the invention provide the advantage that heart rate can be calculated considerably more reliably in an environment comprising interference. The apparatus implementing the method is simpler, faster and less expensive in structure than the prior art solutions.